The present invention relates to a method for forming a film by a CVD (Chemical Vapor Deposition) method on a surface of an object to be processed.
In a film forming process of a semiconductor device, a metal-type thin film is deposited on the surface of a semiconductor wafer to provide a layer structure including a wiring layer, etc. In order to achieve an electrical connection between the wiring layer and the device or between such wiring layers, a contact hole between the device and the wiring layer is filled with a deposited thin film or a via hole between such wiring layers is filled with a deposited thin film. In the case where the via hole is filled with a tungsten film for instance, if any direct contact. occurs between the tungsten and such a wiring layer of, for example, aluminum, there occur a greater contact resistance and poor contact between the wiring layers resulting from a migration effect. In order to avoid such an inconvenience, barrier metal (barrier layer) composed of, for example, a Ti film or TiN film is formed on the inner wall surface and bottom surface of the via hole before the via hole is filled with the tungsten film.
Conventionally, such metal type thin film was formed by a PVD (Physical Vapor Deposition) method. Since a design rule becomes stricter with a recent miniaturization and high integration density of a semiconductor device, a line width and hole's opening size become correspondingly smaller and smaller and the aspect ratio becomes outstandingly higher. As a result, it has been difficult to form a desired film by the PVD method. It has also been very difficult to form such a barrier layer, by the PVD method, on the inner surface of a via hole of a small opening size in particular.
In recent years, it has been a practice to form a barrier layer by a CVD method. In the case where the Ti film is deposited by the CVD method in the via hole to provide a barrier layer, TiCl.sub.4 (titanium tetracheoride) and H.sub.2 (hydrogen) are used as a reaction gas. Further, in the case where the TiN film is deposited by the CVD method in the via hole, the TiCl.sub.4 and NH.sub.3 (ammonium) or an MMH (monomethylhydrazine) are used as a reaction gas.
In the case where a thin film is deposited by the CVD method on a semiconductor wafer in a processing chamber, it is deposited also on a susceptor, on the inner wall, etc., of the processing chamber. After completing the formation of a film but before forming the next film, cleaning is carried out. As the method for effecting cleaning in the processing chamber, use is usually made of a dry cleaning method. In the dry cleaning method, for example, the inside of the processing chamber and susceptor are heated and, at the same time, a ClF.sub.3 gas is introduced into the chamber to decompose a deposited thin film (hereinafter referred to as a deposit). Or an NF.sub.3, SF.sub.6, C.sub.2 F.sub.6, or etc., gas is introduced into the chamber and, by using these, a plasma is created to decompose the deposit. Or the deposit is removed mechanically.
In the case where such dry-cleaning is effected in the processing chamber, particles are increasingly deposited as a cleaning residue in the chamber due to the suction of the residue in the gas supply pipe. Therefore, if the next film is formed after heating the susceptor up to a film forming temperature immediately following the dry-cleaning step, the particles are deposited on the semiconductor wafer, thus failing to achieve formation of a desired film. In the prior art technique, therefore, after the dry-cleaning step but before the next film forming step, a film is formed on a plurality of (for example, 5) dummy wafers. That is, such dummy wafers are used so as to avoid an adverse effect by the particles resulting from the cleaning residue. In this method, however, such dummy wafers are used and there is a rise in cost by that extent. Further, it takes an additional time for forming a film on the dummy wafers, so that the throughput is lowered.